1. Field of the Invention
The present invention relates, in general, to a power steering system for construction equipment and, more particularly, to a simple steering system capable of precisely controlling the position of left and right steering cylinders, thus quickly steering construction equipment with high precision.
2. Description of the Prior Art
The applicant of this invention discloses "a power steering system for construction equipment" in Korean Patent Application No. 96-29,391 (applied on Jul. 19, 1996).
FIG. 1 is a circuit diagram showing the construction of the above power steering system, which is preferably used with a power loader. FIG. 2 is a sectional view of a directional control valve of the steering system of FIG. 1.
In order to steer the loader to the right, a steering wheel (not shown) is rotated by an operator to the right. When the steering wheel is rotated to the right as described above, a steering column 4 is rotated in the same direction. Therefore, a nut sleeve 8, which gears into the steering column 4, is rotated, thus moving the spool 3 of a directional control valve 2 to the left in FIG. 2 within a stroke, thus bringing the valve 2 into a first position (I). When the directional control valve 2 is in the first position (I), the pump line PL is connected to the first cylinder line CL1 through the internal passage I1 of the valve 2, thus feeding pressurized oil from an oil pump into both the small chamber of a right steering cylinder 1a and the large chamber of a left steering cylinder 1b. On the other hand, pressurized oil is discharged from both the small chamber of the left steering cylinder 1b and the large chamber of the right steering cylinder 1a and passes through in the order of the second cylinder line CL2, the internal passage I2 of the valve 2 and a line ML1 prior to being introduced into a reversible motor 5. The pressurized oil, thus rotates the motor 5 in a direction. The rotating force of the above motor 5 is transmitted to the spool 3 through a spline sleeve 11, thereby rotating the spool 3. The spool 3, which engages with the nut sleeve 8, thus precisely returns to the right in FIG. 2 within the stroke, thus bringing the directional control valve 2 into a neutral position (N). Thereafter, the pressurized oil from the reversible motor 5 is fed into the control valve 2 through a line ML2 prior to being returned to an oil tank T through a tank line TL2. It is thus possible to steer the power loader to the right as desired.
Meanwhile, in order to steer the loader to the left, the steering wheel is manually rotated to the left. When the steering wheel is rotated to the left, the steering column 4 is rotated in the same direction. Therefore, the nut sleeve 8 moves the spool 3 of the valve 2 to the right in FIG. 2, thus bringing the valve 2 into a second position (II). When the directional control valve 2 is in the second position (II), the pump line PL is connected to the second cylinder line CL2 through the internal passage II1 of the valve 2, thus feeding pressurized oil into both the large chamber of the right steering cylinder 1a and the small chamber of the left steering cylinder 1b. In this case, pressurized oil is discharged from both the large chamber of the left steering cylinder 1b and the small chamber of the right steering cylinder 1a and passes through in the order of the first cylinder line CL1, the internal passage II2 of the valve 2 and the line ML2 prior to being introduced into the reversible motor 5. The pressurized oil thus rotates the motor 5 in the opposite direction. The rotating force of the above motor 5 is transmitted to the spool 3 through the spline sleeve 11, thus rotating the spool 3. The spool 3 in the above state precisely returns to the left in FIG. 2 within the stroke, thus bringing the directional control valve 2 into the neutral position (N). The pressurized oil from the reversible motor 5 is fed into the control valve 2 through the line ML1 and passes through the internal passage II3 of the valve prior to being returned to the oil tank T through a tank line TL1. It is thus possible to steer the power loader to the left as desired.
However, the above steering system is problematic in that when it is necessary to change the steering direction of the loader from the right or left to the left or right, the spool of the direction control valve, which is in the first or second position, has to be returned to the neutral position, with the steering wheel being stopped, prior to steering the loader to a desirable direction. Therefore, the steering system is time consuming and requires a provision of a complex hydraulic circuit.